Information transmission via an optical fiber transmission line has quite recently been found practicable. It is theoretically possible that a single optical fiber transmission line of a diameter not much larger than a human hair, i.e., 0.025-0.125 mm, corresponding to 0.001"-0.005", has several times the information capacity for carrying all of the television channels from channel #2 through channel #83 inclusive, plus the whole of FM broadcasting, corresponding to a bandwidth of nearly one billion cycles per second (1 GHz).
However, conventional AM or digital (time-division multiplexing) methods cannot efficiently utilize such a large information capacity. A great majority of known experiments with wide band and multiple channel transmission reported in the literature have up until now utilized digital techniques.
While digital techniques have their own unique advantages, the bandwidth requirement therefore is relatively enormous. Thus, for example, a standard TV signal with a signal bandwidth of 6.0 MHz requires an 88 mega bits digital signal to transmit. This is equivalent to a signal bandwidth of more than 200 MHz, corresponding to a bandwidth utilization efficiency of only about 3%.
The standard AM (amplitude modulation) creates two sidebands. If FDM (Frequency Division Multiplexing) is used on AM, the bandwidth requirement increases rapidly as more information bands are added. In addition, because of the non-linearity of the optical detector, intermodulation between bands becomes a very serious problem. Therefore, the standard AM cannot be efficiently transmitted in optical fiber communication. Also, at the present time, standard FM (Frequency Modulation) cannot be easily implemented in optical fiber transmission.
Standard SSB (Single-Side-Band) transmission is known to have a very high efficiency in utilizing the bandwidth. However, conventional SSB techniques used in radio frequency (up to microwaves) cannot be conveniently used at optical (visible, infrared, ultraviolet) frequencies.